Thin, flexible sintered structures, such as sintered ceramic sheets, have a wide variety of practical applications. For example, flexible sintered structures may be used in electronic and/or electro optic applications, such as waveguides, or as substrates for electronic coatings, superconductors, or high temperature superconductors. Flexible sintered ceramic structures may also be useful as a protective layer for glass or other substrate materials where a layer of protection is needed to resist scratching of the substrate. More notably, flexible sintered ceramic sheets or tapes may also be used as an electrolytic catalyst in solid oxide fuel cells.
Current methods for producing thin, flexible sintered ceramic sheets include casting a ceramic slip onto a carrier film to produce a green ceramic sheet or tape. The ceramic slip may be applied to the carrier film using a slot die, roll coater, doctor blade, comma bar or other, similar devices and techniques. The ceramic slip is then dried thereby forming a green ceramic sheet or tape on the surface of the carrier film. Thereafter, the ceramic sheet or tape is removed from the film and fired thereby forming a thin, flexible sintered ceramic sheet or tape.
Producing thin, flat sintered ceramic sheets of area greater than about 100 cm2 having a thickness less than about 45 microns, without resorting to pressure during firing, requires a unique combination of shrinkage and process stress control strategies. The key is to deliver a uniform material at each step in the process. Without this uniformity, an unconstrained body will curl during firing. Non-uniformity in the green body, or in firing, of only 1% from the top of the green sheet to the bottom of the green sheet can yield a curl radius of about 3 mm for a 20 micron thick zirconia electrolyte sheet if all the non-uniformity is converted to differential strain/shrinkage during sintering. Due to “back-stresses” during sintering, all the non-uniformity will not be converted into differential strain and hence curl or warping. Under such conditions, the level of differential shrinkage and curl or warping will yield a curl radius on the order of about 3-10 mm for a 1% shrinkage variation. To obtain thin flat electrolyte sheets, density/sintering shrinkage variations of significantly less than 1% are desirable. However, the application of constraints, such as stacking with release powder, introduces either residual release powder on the body or defects within the body. Such techniques have been applied to thicker bodies. The goal of the present invention is to provide a commercially viable approach that produces large area, thin electrolyte sheet in high yield with minimal defect.
While the aforementioned techniques are suitable for forming thin, flexible sintered ceramic structures, defects such as cracks or holes introduced in the green ceramic sheet or tape during the casting process may render the green ceramic sheet or tape unsuitable for further processing.
Accordingly, a need exists for improved methods and apparatus for casting green ceramic sheets for use in forming thin, flexible, sintered ceramic sheets.